With the increasing demand on high-rate wireless multimedia services and the increasing scarcity of wireless spectral resources, research on future highly-efficient mobile communications system will become more and more significant and meaningful. In order to overcome influences of wireless mobile channel time-varying and multipath fading on signal transmission, error control methods such as forward error correction (FEC) coding and automatic retransmission request (ARQ) are introduced to reduce error rate of the system and ensure quality of service. Although the time delay in FEC scheme is small, code redundancy present therein reduces system throughput. Although an ideal throughput can be achieved in ARQ when there is not much error rate, time delay generated in ARQ is relatively great, thereby making it unsuitable in providing real-time services. To overcome the deficiencies of the two schemes, incorporation of the two methods engenders the Hybrid Automatic Retransmission reQuest (HARQ) scheme: i.e. an FEC subsystem is contained in the ARQ system, whereby when error correcting capability of the FEC is capable of correcting the errors, the ARQ is not used; and only when it is impossible for the FEC to properly correct errors, the retransmission of the erroneous codes is requested via the ARQ feedback channel. Effective incorporation of ARQ with FEC not only provides higher reliability than in the case of the single FEC system but also provides higher system throughput than in the case of the single ARQ system. Accordingly, with the rapid development of higher data rate or higher reliability for services, HARQ has become an essential technique in wireless communications system and has been studied in-depth.
Classification of HARQ Technology
According to different classification modes, HARQ can be classified into synchronous HARQ technique and asynchronous HARQ technique, non-adaptive HARQ technique and adaptive HARQ technique, and into different retransmission types and retransmission mechanisms.
Synchronous HARQ technique and asynchronous HARQ technique: HARQ can be classified into synchronous type and asynchronous type according to timings at which retransmissions occur. Since the occurring timing of transmission is known to the receiving end in advance in synchronous HARQ, the serial number of HARQ process can be obtained from the subframe number. Transmission of asynchronous HARQ process can occur at any timing, and the processing serial number of HARQ process should be transmitted together with the data. Although asynchronous adaptive HARQ technique has higher flexibility than synchronous non-adaptive technique in terms of scheduling, less signaling cost is required in the latter.
Retransmission types: according to different contents to be retransmitted, HARQ mainly includes three types of hybrid automatic retransmission request mechanisms, which are respectively referred to as HARQ-I, HARQ-II and HARQ-III. FEC coding and CRC check are performed in all three types in common, FEC decoding and CRC check are performed at the receiving end, and retransmission is requested in case of error in the packets. The three types differ from one another as follows: in HARQ-I erroneous packets are discarded, packets to be retransmitted are the same as the already transmitted packets, and there is no combination decoding; in HARQ-II erroneous packets are not discarded but decoded in combination with the retransmitted packets, and the packets to be retransmitted may differ from the already transmitted packets both in format and in content; since CPC codes (complementary punctured convolutional codes) are employed in HARQ-III, each transmitted packet and each retransmitted packet can be decoded by itself, and each retransmission may generate different redundancies (different bit puncturing) and may also generate identical redundancies (identical FEC), in which case the operation is similar to that in HARQ-I, but erroneous packets should be stored at the receiving end to facilitate incorporation with the packets to be retransmitted.
Adaptive and non-adaptive: HARQ can be further classified as non-adaptive type and adaptive type according to whether data characteristics change during retransmission, wherein data characteristics as transmitted include assignment of resource blocks, mode of modulation, length of transmission block and duration of transmission. Adaptive transmission means that the transmitting end can change partial transmission parameters in accordance with practical channel status information during each process of retransmission. Accordingly, the control signaling information that contains transmission parameters should be transmitted together during each round of transmission, thus causing additional signaling cost. Changeable transmission parameters include mode of modulation, assignment of resource units and duration of transmission, etc. However, these transmission parameters are known to the receiving end in advance in the non-adaptive system, so that signaling is relatively simple.
Three standard protocols in the traditional automatic retransmission request (ARQ) are SAW (stop-and-wait) ARQ, GBN (go-back-n) ARQ, and SR (selective repeat) ARQ. ARQ and HARQ can both be used either in an FDD system or in a TDD system.
In a general HARQ system, when the receiving end detects an erroneous data packet, the failed bits usually occupy only a part of the data packet, so that retransmission of the entire data packet will engender certain loss to the throughput. Both the long term evolution (LTE) of 3GPP and the enhanced type (802.16m) of the WiMAX system select HARQ-II and III as optional solutions. When the first attempt of decoding fails, the transmitter adds redundant information or processes again for transmission, the retransmitted packet is not completely identical with the originally transmitted packet, and incorporation of the retransmission information with previously received data packets will get better system throughput.
FIG. 1 is a view schematically illustrating the structure of a HARQ system. As shown in FIG. 1, the HARQ system comprises a transmitting unit 100, an ARQ controller 101, a modulation and coding scheme (MCS) controller 102, a wireless channel 103, a channel estimator 104, a receiving unit 105, a modulation and coding scheme selector 106, and an ARQ check 107. In addition, data cache and reception cache are usually included. The data cache is used to temporarily store data to be transmitted or data already transmitted but not checked as correct, and the reception cache is used to temporarily store received data. The data cache, transmitting unit, ARQ controller, and MCS controller as shown in FIG. 1 constitute the transmitter section of the HARQ system. The transmitter can for instance be a base station (node B) in a wireless communications system, and can also be a server in a general network (such as the Internet or an LAN). The channel estimator 104, receiving unit 105, modulation and coding scheme selector 106, ARQ check 107, and the reception cache as shown in FIG. 1 constitute the receiver section of the HARQ system. The receiver can for instance be a mobile station in a wireless communications system, and can also be a personal computer connected to a server in the Internet or an LAN. In other words, the HARQ system as shown in FIG. 1 can be applied in a wireless communications network, and can also be applied in a wired network. The HARQ system can for instance be applied in the TCP/IP network.
The general processing procedure of the HARQ system is explained below with reference to FIG. 1.
Firstly at the initial state, the transmitting unit 100 at the transmitter section (transmitting end) modulates and codes data to be transmitted in the data cache in accordance with modulation and coding information provided by the MCS controller 102, and transmits via an antenna, for instance, new data packets generated by modulating and coding the data to be transmitted.
The receiving unit 105 at the receiver section (receiving end) receives the data transmitted by the transmitting unit 100 via the channel 103, and the received data is performed with CRC check by the ARQ check unit 107. If it is checked as correct, the correctly received data bits are outputted, and an ACK signal is returned to the ARQ controller 101 at the transmitting end; otherwise, a NACK (also referred to as NAK) signal is returned, and information (such as bit soft information) of the current data packet is retained in the reception cache. At the same time, the MCS controller 106 at the receiving end determines the modulation and coding scheme by calculating such parameters as the effective signal-to-noise ratio in accordance with the result of channel estimation, and feeds back the same to the MCS controller 102. Feedbacks of MCS and ACK/NACK signals are two independent branches, and the feedback frequencies thereof can either be identical with or different from each other depending upon system setup or channel environment. When the receiver returns to the initial state, the data is coded and modulated in accordance with the feedback MCS.
Thereafter at the transmitter section, upon receipt of the ACK feedback or NACK feedback returned from the ARQ check unit, the ARQ controller firstly determines whether the received feedback is ACK or NACK. If what is received is ACK, the initial state is returned, and the data is coded and modulated in accordance with the feedback MCS to continue transmission of new data packets. If what is received is NACK, the number of retransmission is added by 1, and when the number of retransmission does not exceed the preset maximum number of retransmission, the data packets transmitted last time are transmitted again. The format of the retransmitted data packets (such as the coding and modulation mode, and sizes of the data packets, etc.) can either be the same as that during the first transmission (namely Chase Combining) or selected as a new packet format in accordance with the latest MCS feedback (namely IR incremental redundancy mode of HARQ-III type). When the number of retransmission reaches an upper limit, the initial state is returned to continue transmission of new data packets.
Upon receipt of the retransmitted data packet, the receiver section incorporates the new information with information retained in the reception cache to decode again (incorporation of the retransmitted data information with the retained information can effectively reduce error rate and enhance throughput). The decoded data is performed with CRC check, and the check result is then fed back (as ACK/NACK) to the transmitting end.
As can be seen, the transmitting end transmits new data only when the ARQ controller 101 receives the ACK signal or the maximum retransmission number is reached. Thus, the transmitting end and the receiving end both require certain cache space to store the data not correctly transmitted.
FIG. 2 is a view schematically illustrating HARQ data frames and the time sequence of retransmission. In the example as shown in FIG. 2, each data frame includes a plurality of data packets (4 data packets in the example as shown). The receiving end (receiver section) feeds back one CRC check result to the transmitter section (transmitting end) for each data packet. In the example as shown in FIG. 2, of the four data packets P1, P2, P3 and P4 as initially transmitted, P1 and P4 are known for instance through CRC check to have been correctly received, so that feedbacks with regard to them are ACK. Whereas feedbacks to P2 and P3 that have not been correctly received are NACK. Consequently, as shown in FIG. 2, data packets P2 and P3 are retransmitted in the next frame, while other positions in the frame can be used to transmit new data packets (new data packets P5 and P6 as shown in FIG. 2). T as shown in FIG. 2 is the length of the data frame, and Td indicates inter-frame space.
As can be seen from FIGS. 1 and 2, in the conventional technology, the HARQ system carries out retransmitting process on the entire data packets (PDU) of the MAC layer, and each retransmission occupies relatively great channel resources.
The data packets are performed with CRC check as a whole at the receiving end in the general HARQ retransmission mechanism, whereas situation in the practical system might be that only few bits are erroneous, so that retransmission of the entire (or partial) packets occupies considerable channel resources. A retransmission method based on coded block has been proposed to further improve HARQ performance. In this method one data packet consists of several coded blocks carrying their own check codes, and data retransmission takes the coded block as the minimum unit.
FIG. 3 is a schematic view illustrating the structure of a data packet according to the technical solution of coded block retransmission. The structure of the data packet as shown in FIG. 3 is merely exemplary in nature, as it may include more or less coded blocks. As shown in FIG. 3, the data packet 401 (also referred to as Transport Block, TB) in the frame includes several (four as schematically shown in FIG. 3) coded blocks 402 each carrying a CRC check code 403. After passing through the coder, one source data sub-packet corresponds to a combination of a coded block and a CRC check code. The whole data packet can be finally added with a CRC check code 404, and can also not be added with the check code. The receiving end checks each coded block, and if there is an error, only the erroneous coded block is retransmitted in the next packet of data, while other coded blocks can be placed with new data, thus avoiding the problem of having to retransmit the entire data packet in the general HARQ technology.
In the technical solution based on coded block retransmission, the coded blocks replace the entire data packet as the minimum unit for retransmission, thereby greatly improving system throughput. However, a coded block can reach as much as 6144 bits in the 3GPP LTE system, and this means the load of retransmission is still burdensome.
As should be noted, the above explanations to the conventional technologies are merely to facilitate lucid and complete explanations to the technical solution of the present invention, and to facilitate comprehension by persons skilled in the art. These technical solutions should not be regarded to have been publicly known to persons skilled in the art only because they have been explained in the Background of the Related Art in the present invention.
Reference documents relevant to the present invention are listed below and incorporated herein by reference, as if they were described in detail in the present Description.    1. [Patent Document 1]: Wu, et al., Adaptive multi-mode HARQ system and method (U.S. Pat. No. 7,152,196 B2);    2. [Patent Document 2]: Stewart, et al., Block puncturing for turbo code based incremental redundancy (US 20070061690 A1);    3. [Patent Document 3]: Mo, et al., Packet transmission apparatus and method using optimized punctured convolution codes (US 20070234186 A1);    4. [Patent Document 4]: Qiu, et al., Wireless terminal turbo decoding module supporting packet splitting and decoding (US 20070280158 A1);    5. [Non-Patent Document 1]: 3GPP TR25.835. Report on hybrid ARQ type II/III [S]. 2000;    6. [Non-Patent Document 2]: C. Bai, B. Mielczarek, W. A. Krzymie'n, and I. J. Fair, “Sub-block recovery scheme for iterative decoding of turbo codes” in Proc. IEEE VTC'05-Fall, Dallas, USA, September 2005;    7. [Non-Patent Document 3]: Tao Shi; Lei Cao, “Combining techniques and segment selective repeat on turbo coded hybrid ARQ”, in Proc. IEEE Conf. WCNC. 2004 IEEE, Vol. 4, pp. 2115-2119, 21-25 Mar. 2004.